1. Field of the Invention
The present invention relates to an image forming apparatus capable of variably controlling the rotation speed of a rotating polygonal mirror for deflecting a laser beam.
2. Description of the Related Art
In recent years, image forming apparatuses, such as copying machines and printers, capable of forming images on a plurality of types of sheets at high speed, with high image quality, and with high accuracy have been in demand. In particular, to increase the speed of forming color images, a plurality of image forming units corresponding to four colors of yellow, magenta, cyan, and black may be utilized by a color image forming apparatus to serially form color images.
Such an image forming apparatus uses a laser beam (a beam generated by a beam generation unit) and a drive motor that drives a rotating polygonal mirror (a scanning unit) for deflecting the laser beam (hereinafter referred to as a “scanner motor”). In order to allow such an image forming apparatus to operate at high speed, various methods for increasing the speed of forming images have been proposed. In addition, a method has been proposed in which an interval between images serially formed (a page interval, namely, an interval between print sheets) is controlled to be small (not to make a large interval between images).
Meanwhile, in order to form a high quality image, it is desirable to accurately align a position of an image formed in a print sheet. Furthermore, it is desirable to reduce color drift occurring at the time of superposing images of a plurality of colors and to reduce density variation of an image formed on a sheet.
Characteristics or attributes of print sheets to be used for forming images thereon differ depending on use applications (one-sided printing and two-sided printing), use conditions and environments (humidity and temperature), sheet thickness and hardness, and sheet manufacturers. Accordingly, the amount of thermal contraction of a print sheet that has passed through a fixing device to heat and fix an image formed on the print sheet differs with each type of print sheet.
Thus, it is desirable to accurately align a position at which an image is formed on a print sheet according to the amount of thermal contraction of the print sheet. In this regard, in a conventional method, the magnification of an image (the size of an image) is appropriately changed according to characteristics and attributes of print sheets. Japanese Patent Application Laid-Open No. 2002-354234 discusses a method for changing the magnification of an image by controlling the rotation speed of a scanner motor to correct shifting of a position at which an image is formed.
The temperature of a photosensitive drum (photosensitive member) for each color and an intermediate transfer belt (intermediate transfer member) varies during image forming. In addition, after a continuous use thereof for a long period of time, a photosensitive drum and an intermediate transfer belt can be deteriorated. These environmental matters need to be considered or addressed in order to form a high quality image. Moreover, shifting of an image forming position occurring due to variation in environmental matters needs to be reduced to be sufficiently small. In order to reduce shifting of an image forming position, an ordinary image forming apparatus forms a position detection mark (registration patch) to determine the amount of shifting of an image forming position by reading the formed position detection mark with a sensor, and to correct a timing for writing a latent image during subsequent image forming.
In addition, in a conventional method, an image density adjustment mark (density patch) is formed in order to maintain the density of images formed during serial printing to be constant, and the density of an image to be formed is corrected based on the density of the image density adjustment mark read with a sensor, so as to reduce density variation occurring due to environmental variations.
In the method discussed in Japanese Patent Application Laid-Open No. 2002-354234, the rotation speed of a scanner motor is changed at a page interval (interval between print sheets) according to the image magnification to form an image page by page. In this method, image forming cannot be started during changing of the rotation speed of the scanner motor (during switching of the rotation speed). Accordingly, forming of the next image cannot be started until the changing of the rotation speed is completed (until the rotation speed is completely switched). In this regard, several hundreds of milliseconds, for example, is required for a time period from the start of changing of the rotation speed to the end thereof depending on the type of a scanner motor used in image forming.
Even in a case where the rotation speed of a scanner motor is not changed, in forming various patches at a page interval, if the edge portion of a print sheet and a patch are positioned too close to each other, the edge portion of a print sheet can be stained due to scattering of a developer (toner) occurring during forming of a patch. Accordingly, it is necessary to form a patch image (pattern image) with a predetermined interval set between print sheets.
Furthermore, in a case where changing of the rotation speed of a scanner motor and forming of a registration patch or density patch are performed at the same page interval, it is necessary to form the registration patch or density patch during a time period in which changing of the rotation speed of the scanner motor is not being performed. Accordingly, a page interval needs to be further enlarged. Thus, productivity is adversely lowered in forming a high quality image.